1. Field of the Invention
The present invention relates to a multi-tone image processing method and apparatus for use in, for example, electrophotographic digital copying apparatus and printers.
2. Description of the Related Art
Various recording methods have been proposed for conventional digital copying apparatus and printers, such as a density pattern method, a dither pattern method and the like for recording multi-tone images.
The density pattern method binarizes original image data of a single picture element by review of a plurality of threshold values associated with a plurality of recording dots, so as to reproduce a plurality of tone levels by a plurality of mutually dissimilar dot patterns. The dither pattern method binarizes original image data of a single picture element by review of one of a plurality of threshold values associated with a plurality of recording dots, so as to reproduce a single picture element of the original image by a single recording dot. A density-dither pattern method combining both the density pattern method and the dither pattern method are also known.
The matrices of threshold values used in the aforesaid methods can be classified as Fattening types wherein dots are concentrated in the center of the matrix in halftone levels, and Bayer types wherein dots are dispersed throughout the entire matrix in halftone levels. When a Bayer type threshold value matrix is used, there is a tendency to have a high degree of gamma recording characteristics. On the other hand, when a Fattening type threshold value matrix is used, resolution is reduced when the matrix size is increased to increase the number of tone levels, and the number of tone levels is diminished when the matrix size is reduced to improve resolution.
An improved halftone method (hereinafter referred to as “IH method”) has been provided as a recording method that reconciles both tonality and resolution, wherein a transformed Fattening type threshold value matrix is used, and the sizes of the dots are altered by pulse width modulation, as shown in FIG. 1 (refer to Electrophotography, The Society of Electrophotography of Japan, Vol. 25, No. 1, 1986; pp. 31-44). When the aforesaid IH method is used, however, the output characteristics unavoidably include a high degree of gamma characteristics. Thus, only the necessary patterns must be extracted to equalize the steps of density variation so as to obtain linear output. In such an instance, the actual number of reproducible tone levels is less than the theoretical value.
In the previously described methods, it is difficult to achieve adequate compatibility between tonality and resolution. This difficulty is due to the precondition that the frequency of recordings within the matrix range, i.e., the total recording time, must be invariably increased to improve the output density.